It is generally known that a linear polyester resin prepared by condensation polymerization of polyol with dicarboxylic acid has been used as a resin for a toner because of its sharp melting property, of which details are disclosed in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP O.P.I. Publication) No. 50-63939/1975.
However, there exists a problem that when fixing the color toner comprising the above linear polyester resin with a heat-roller, an offset phenomenon is liable to take place by sticking of the fused toner to the heat roller.
There is known the technique for preventing an offset, in which sticking of the melted toner to the heat roller is prevented by coating silicone oil on the surface of the heat-roller.
In the above techniques, there are involved such incoveniences particularly in the case where the fixed image is formed on an OHP sheet comprising a polyester film, that the OHP sheet is contaminated by silicone oil adhering in a thin layer on the surface of a fixed image, which in turn results in contaminating fingers of an operator and other OHP sheets.
There is proposed another method for providing an antioffset property to a toner itself in JP O.P.I. Publication No. 57-37353/1982, in which the toner comprises the linear polyester resin having a crosslinking structure formed by using tricarboxylic acid in combination with diol and dicarboxylic acid.
A dye or a pigment contained in a color toner is sometimes liable to adversely affect a triboelectricity of a toner and therefore, a colorless charge controlling agent is added to the toner, of which examples are the organic metal complexes disclosed in JP O.P.I. Publication Nos. 57-104940/1982 and 57-124357/1982.
However, some of the above organic metal complexes have high reactivity to a polyester resin having a crosslinking structure, and combined use of such high reactive organic metal complexes and the polyester resins having the crosslinking structure were proved to cause the following problems:
(1) a charge control function of the organic metal complexes is spoiled, so that an image density is lowered and a background fog is produced; PA1 (2) fixing is deteriorated by increase in the softening point of a toner; PA1 (3) the incresed softening point of the toner deteriorates dispersibility of a colorant in a polyester resin, which in turn results in making a hue of a fixed image less sharp and lowering color developability of a multicolored image formed by superposing the colors because of lower transparency of the toner; and PA1 (4) lower transparency of an image formed on a transparent OHP sheet makes it difficult to project a sharp OHP image. PA1 (1) a color image having a high density and no background fog can be formed; PA1 (2) an excellent fixation with a heat roller can be achieved without causing any offset phenomena; PA1 (3) there can be formed a color image having a clear hue, a high transparency and an excellent color developability due to an excellent dispersibiliy of a colorant; and PA1 (4) a sharp and clear color image having a high transparency can be formed on an OHP sheet. PA1 Component (1): diol represented by the following Formula (a): ##STR2## wherein R represents an ethylene or propylene group; and x and y are each an integer, provided that the sum of x and y is 2 to 7 in average; PA1 Component (2): the mixture of a divalent acid component comprising a divalent carboxylic acid having the aliphatic hydrocarbon side chain of 3 to 22 carbon atoms or the lower alkyl ester thereof, and a trivalent acid component comprising a trivalent carboxylic acid or the anhydride thereof in the proportion of 10 to 50 mol % of the total acid components.
At present, it is the actual situation that there are available no organic metal complexes compatible with the polyester resin having the crosslinking structure.